1. Field of the Invention
The present invention relates in general to the cementing of a prosthesis in a bone cavity section and specifically to a sensor for enabling the viscosity of a flowable cement to be detected as it is pressurized into the bone cavity section so as to prevent the flowable cement from passing through the intertrabecular spaces in the bone wall into the bloodstream of the patient.
2. Description of Related Art Including Information Disclosed Under 37 CFR 1.97 and 1.98
In commonly owned U.S. Pat. No. 4,357,716, incorporated hereby reference in its entirety, there is disclosed a system for mounting a femoral stem prosthesis in the femoral canal with the use of a cement.
The process includes sealing the bone cavity section in a substantially fluid-tight relationship, inserting the cement through a seal into the bone cavity under pressure, having an air escape orifice that allows air and body fluids to escape the bone cavity until it is full of cement, plugging the air escape cavity, and holding the pressure on the cement until it sets.
There are other methods of pressurizing bone cavities filled with cement. Some of them use orifices in the prosthesis itself to inject the cement under pressure.
Presently, the major compounds of bone-cement consist of a liquid monomer of methylmethacrylate, a powder, polymethacrylate, and radiopaque barium sulfate. The liquid and powder may be mixed by one of several methods and then deposited into a gun or syringe having a long nozzle. The cement is injected in a retrograde fashion, i.e. from the bottom up to near the top of the femoral canal. The initial injection of cement is somewhat timely. It should begin when the mixture is in the early doughy state. The cement should be of such viscosity that the femoral canal can be satisfactorily filled without entrapment, or at least with minimal entrapment, of air pockets. At the same time, the cement should be of such viscosity that it will partially fill the intertrabecular spaces but not flow from the intertrabecular spaces into the veins beyond the cortical bone. Usually the mixture changes from a liquid state to an early doughy state at about three minutes from beginning to mix the liquid monomer and the powder polymer.
The change from the stage when the mixture is a liquid to a slightly doughy state usually occurs in a matter of a few seconds. That period of time varies somewhat with different brands of cement. Other factors also influence the speed of this reaction or change. A variation of six degrees temperature markedly alters the speed of the reaction. A higher temperature increases the speed. Therefore, the room temperature, the overhead operating room lights, the temperature of the vial and monomer, the temperature of the package powder and the mixing bowl and the stirring ladle all influence the speed of the reaction.
Further, the method and speed of mixing affect the speed of the reaction. The greater the amount of oxygen that is mixed with the cement the more the speed of the reaction will be decreased. Most mixing is done in a container with a vacuum attached to help remove ambient vapor from the operating room. The vacuum pulls air into the mixing bowl and so the length of time that the vacuum is attached and the amount of suction influences the amount of oxygen that will pass over the mixture as well as the degree of evaporation. Further, the monomer-polymer ratio may also have been altered which affects the speed of the reaction.
The fact is, it is extremely important for the physician to monitor the viscosity of the cement so that, when it is still in a highly liquid state, it will not be forced under pressure through the intertrabecular spaces to the veins lying beyond the cortical wall.
Presently, the physician checks the viscosity by extruding a small amount of the cement on paper and a second bit of the cement between gloved fingers. The cement is considered to be of acceptable viscosity when it no longer sticks to the glove. However, this testing is not totally reliable. Probably the two most common undetected variations are (1) the sample piece of cement does not have the same homogeneity as the cement that is injected into the canal and/or (2) the cement is cooler than normal and so the cooler cement does not have as great an adhesive property when tested as it would have at a higher temperature.
There are two methods of pressurizing cement in the femoral canal. The first method pressurizes the cement before the femoral stem is inserted and the canal is nearly filled with cement in retrograde fashion. Then, a cannulated plug, with the nozzle of the cement gun passing through the plug, is placed into the proximal end of the femoral canal and cement is applied under pressure for a limited time, usually about twenty seconds. The physician applies the amount of pressure which "feels right". It is obvious that the amount of pressure which "feels right" may vary for a physician from day-to-day as well as from one physician to another.
At the beginning of pressurization, the physician needs to know that the cement in the canal is of sufficient viscosity that it will not flow through the Haversian canals and intertrabecular spaces into the veins beyond the medullary canal and cortex.
There is also a need to know the maximum amount of pressure that can be applied to extrude the polymerizing cement without forcing the cement into the veins. The pressure which is exerted with the cannulated plug in place is estimated to be approximately 25 psi but, of course, may vary.
If the bone cavity is filled with cement under pressure first, the cannulated plug and cement gun are then removed and the prosthetic stem is then manually inserted. This temporarily increases the pressure in some areas, particularly on the concave side of the stem as the stem displaces the cement. As the stem is manually introduced, there is lateral or radial micromotion of the stem and displacement of cement. It is desired that the cement return to the immediate adjacent area of the stem from which it was displaced to form a good cement/stem connection. The return of the cement is good with cement of low-level viscosity, but diminished if the cement is moderately doughy. Then when the cement begins to expand, due to exothermic reaction, pressure is increased and some cement may rise above the femoral canal. The cement then begins to cool and shrink for about five minutes. The resultant pressure is markedly decreased after the cement has cooled and is mature. The end result is essentially that of having filled the canal and having slightly packed the cement.
The second method of pressurizing cement in the femoral canal or other bone cavity is that of inserting the prosthesis through a sealing device and inserting it in the femoral canal and attaching the sealing device to the canal or bone cavity in a substantially fluid-tight relationship and injecting cement through orifices in the sealing device. This is clearly shown in commonly owned U.S. Pat. No. 4,357,716. The sealing device in U.S. Pat. No. 4,357,716 includes an orifice for venting the canal so that air and other bodily fluids in the femoral canal can be displaced by the cement and escape through the orifice. However, when cement begins to exude from the orifice, it is then closed so that cement can no longer exude and the pressure is increased to compress the cement in an effort to remove or compress air bubbles and to cause the cement to fill the intertrabecular spaces and surround the prosthesis in a binding relationship. The hole from which the cement can exude is a one-quarter inch hole which, of course, allows the fluid products to easily flow therethrough.
Thus, when the orifice is sealed and pressure is applied, if the cement does not have sufficient viscosity, the pressure applied may force it through the intertrabecular spaces and Haversian canals and into the veins on the other side of the cortical wall.
If cement does flow into the veins on the other side of the cortical wall, there may be extremely adverse effects on the patient's health.
Thus, it would be desirable to have a more exact way of determining the viscosity of the cement to which pressure is being applied so that pressure of greater amounts than are needed or desired will not be applied when the viscosity is low and a maximal amount of pressure can be applied when the viscosity has become so great it cannot enter through the intertrabecular spaces.